Merocyanine dyes are heterocyclic chromophores that can be used in a number of applications, for example, as photographic sensitizers, for non-linear optics, and in chemotherapy, to name a few. See, e.g., Chen et al., J. Environ. Pathol. Toxicol. Oncol. 25:217-222 (2006); Yow et al., Toxicol. Lett. 115:53-61 (2000); Zareba et al., Biochim. Biophys. Acta. 1722:51-59 (2005); Marder, Chem. Commun. 131-134 (2006); Marder et al., J. Am. Chem. Soc. 115:3006-3007 (1993); and Brooker et al., J. Am. Chem. Soc. 73:5332-5350 (1951). Merocyanine dyes also have been used as sensors of protein conformation and protein interactions in live-cell fluorescence imaging. See, e.g., Nalbant et al., Science 305:1615-1619 (2004); Toutchkine et al., J. Am. Chem. Soc. 125:4132-4145 (2003).
The efficacy of merocyanine dyes as components of biosensors depends not only on their fluorescence emission properties, but also on their photostability. In many cases, however, merocyanine fluorescent dyes degrade rapidly in light. Thus, the use of many merocyanine fluorescent dyes for live-cell imaging has been limited by rapid photobleaching of the dyes. Thus, there is a need in the art for merocyanine fluorescent dyes having improved photostability.
Further, ratiometric dyes having an excitation/emission wavelength that is dependent on solvent or protein environment are needed for live-cell imaging. A disadvantage of merocyanine dyes known in the art is that their excitation/emission wavelengths are not sufficiently solvent-dependent for use in live cell biosensors. This characteristic is thought to arise because the dyes are at the “cyanine limit.” That is, they have an equal contribution of non-polar and polar resonance forms in the ground and excited states, which results in such dyes having similar dipole moments in the ground and excited states. As a consequence, the dyes' excitation and emission wavelengths are not solvent sensitive. Thus, there is a need in the art for merocyanine dyes having excitation and emission wavelengths that are solvent sensitive, and have sufficient photostability. The presently disclosed subject matter addresses, in whole or in part, these and other needs in the art.